Process of making vitreous silica



June 9, 1925.

L. B. MILLER M00555 0? MAKING vz'razous SILICA "iled Aug. 10, 1921 A m ZHU Y L u M 4 w V 7////////% Patented June 9, 1925.

UNITED STATES I r 1,541,584 PATENT OFFICE.

' LEVI B. MILLER, 01 LYNN, MASSACHUSETTS, ASSIGNOR TO GENERAL ELECTRIC COH- PANY, A CORPORATION OF NEW YORK.

PROCESS OF MAKING VITREOUS SILICA.

Application filed August 10, 1921. Serial No. 491,122.

To all whom it may concern.-

Be it known that I, LEVI B. MILLER, a

perature up to about 550 C., the differences in coefficient of expansion along different axes of the crystal usually causes cracking or shattering of the crystal. higher temperature the pieces resulting from the cracking of the quartz crystal. coalesce by fusion, numerous air bubbles are trapped which are'removable only with the greatest difficulty, if at all. Various methods have been suggested and used in order to avoid the trapping of .air bubbles. ()ne of the most successful of these methods involves the heating of the quartz in convenient form, usually as coarse particles, in a highly evacuated space so as to reduce to a minimum the inclusion of air or other gases at the coalescence of the quartz particles. Some gasbubbles are nevertheless enclosed by the coalescence and fusion of the quartz particles, requiring subsequent manipulation of the quartz fusion in order to remove or reduce the bubbles. For some purposes, particularly for optical purposes, it is sirable to produce fused or vitreous silica which is entirely devoid of bubbles, particularly of the exceedingly minute bubbles which give fused silica a milky appearance.

In accordance with my invention perfectly transparent, clear silica is produced by heating in a vacuum masses of clear qiiartz crystals mechanically confined in such a way as to prevent displacement of the particles resulting from the disintegration of the quartz when passing through the temperature of about 550. Preferably the quartz is thereupon heated as a single operation to a high. enough temperature to fuse the same and coalesce the particles, while thus supported, thereby producing a clear mass of fused quartz.

The accompanying drawing shows appalVhen at a drawing.

of the crystal, as a retus suitable for carrying out my invention, Fig. 1,, showing in vertical section a vacuum furnace in which a mass of crystalline quartz is heated while packed in granih lar or powdered gra hite. Fig. 2 illustrates a modified support or a mass of crystalline quartz.

Referring to the drawings, a vacuum furnace has beenindicated in Fig. 1 somewhat diagrammatically as furnaces of thiscl ass are well known. The furnace comprises'an outer metallic tank 1 having a removable cover 2, the cover being connected to the tank by any suitable mechanical means, indicated as bolts 3, suitable gas-tight -washers 4 being employed. The furnace may be provided with a removable column 5 mounted upon the cover. The column 5 is shown in part broken away as itsconstruction is well understood. A suitable construction is shown in my prior application Ser. No. 457,660 filed April 1, 1921 and in United States Patent No. 1,536,821, is-

sued to Philip K. Devers on May 5, 1925.

The. column may be provided with a. window for viewing the interior of the furnace or through the column a char e'may'be introduced into the furnace. T e furnace is provided with a helical resistor 6 consisting of graphite, or other suitable ma.- terial, and mounted at its opposite ends upon suitable electric terminals 7, 8. The terminals may be water-cooled by ducts 9, 10, the cooling water being supplied by tubes 11, 12, which serve also as supports and current conductors for the resistor; The tubes 11, 12 are supported from the cover of the furnace, suitable electrical insulation being provided as indicated in the Surrounding the resistor 6, is a heating'screen1'3, which conveniently consists of a box of carbon or graphite containing a powdered refractory material, such as charcoal or fine graphite. Upon the pedestal 14 resting upon the lower' closed end of the heater 6 is a crucible 15. In this crucible isfcontained a unitary mass of clear or stalline quartz 16 tightly packed within t e graphite particles 17 so as to prevent mechanical displacement of the crystalline parts resultin from the shattering ve explained. The crucible may be provided with vent holes, as indicated in the drawing, the top being preferably closed by -Qa plug of iroin dis cured to graph te pluglo may he use to re time the crucible and charge in the id to remove t ie seine.

cases when unusual care should to prevent entry of particles of "1:30 crscizs adjacent the surface of quartz, and to more secinely quartz crystal a packing or may he used which is illustrated I this case the mess of crystal.-

shaped by grinding, or nfornto the interior of the a plug 23 is screwed thereto V .iv secured to the walls of ucihle in cont ct with the crystalt'urnace has been charged. on

. l i'lillll-lz muss suppdrtec', by either or shove methods, he interior is evacuated through 1: tube aid the temperature is increa ed as first as permissible without causing locni overheati or damage to the furnace parts. Ho pniticu ut' care need he i heat the crystal any slower or the conversion tempere- 5 than above this temperature. quartz mass has been heated to ..,no 'gh ten'iperuture to cause sottening peruture of about 1650 -1700 C. t may he discontinued, this tern: the crystalline particles will conlescc so that upon cooling or uniform clear 0- structureless mess ct amorphous silica is secured. This clear structureless ninss oi? amorphous silica may be subsequently inunipuluted when heated to a temperature of plasticity or may be mechanically shaped cutting, grinding or otherwise in order to produce lenses or other objects of desired i'orni.

What I claim as new and desire to secure by Letters Patent of the United States, is,-

l. The method of producing vitreous silica which consists in heating a unitary mass of crystalline quartz to a temperature sufliciently high to convert the some into plastic vitrcous'silica While supporting said mass to prevent displacement of crystal particles resulting from the disintegration of said crystalline quartz.

The method of producing clear vitreous silica or quairtz glass which consists in heatquartz in the crystal state to a fusing temperature in u ncuum While mechanically supporting said quartz to prevent displacement of particles resulting from the cracking of said quartz when heated to a temperature of about 550 C.

5-3, The method of producing clear silica glues which consists'in mechanically shaping a unitary mass of clear crystalline quartz to conform to the interior dimensions of n mold, confining the shaped quartz in suid mold and heating said quartz While held in said mold to a temperature suiiiciently high to convert the quartz from the crystalline to the amorphous state.

In Witness whereof I have hereunto set 111) hand this 8th day of August, 1921.

LEVI B. MILLER. 

